1. Technical Field
This invention relates generally to protection circuits for rechargeable battery packs, and more specifically to protection circuits that disable a rechargeable battery pack due to an excessive amount of power being drawn by the load.
2. Background Art
Portable electronic devices, like cellular telephones, pagers and two-way radios for example, derive their portability from rechargeable batteries. Such batteries allow these devices to slip the surly bonds of wall mounted power supplies and wirelessly touch the hand of the user wherever he may be.
While many people may think that a rechargeable battery is simply a cell and a plastic housing, nothing could be further from the truth. Rechargeable battery packs often include circuit boards, electronic circuitry, mechanical assemblies and electromechanical protection components. The circuits employed in rechargeable battery packs include charging circuits that start, ramp, taper and stop current, fuel gauging circuits, temperature measurement circuits and indicator circuits, just to name a few. Simply put, a battery pack is a complex system of components working in harmony to safely deliver power to a portable electronic device.
One of the most fundamental circuits in a battery pack is the protection circuit. Rechargeable battery performance, especially with respect to those having cells constructed of lithium-based materials, may be severely compromised if the cell within the battery pack is over or under charged. For this reason, most all battery packs today include one form of safety circuit or another.
Typical safety circuits include voltage and current limits. As such, when the voltage across the cell in a battery pack becomes too high or too low, the safety circuit will open switches within the pack, thereby “turning off” the battery pack. Similarly, if the current flowing either into or out of the cell gets too high, the safety circuit will turn off the battery pack.
Despite these voltage and current safety mechanisms, new concerns are arising from “over power” situations. These situations arise when a battery pack is operating within its voltage and current limits, but the total power—the product of voltage and current—becomes too high for a particular electronic device. The concern is that the over power situation may cause components within the electronic device to generate excessive heat.
There is thus a need for an improved battery safety circuit that turns off the battery not only due to excessive voltage or current, but for excessive power dissipation as well.